Surface micromachining is the process used to fabricate many MEMS devices. With surface micromachining, the MEMS device structure can be built up on a silicon substrate using chemical vapor deposition to alternately deposit layers of structural polycrystalline silicon (polysilicon) and a sacrificial material (e.g., plasma-enhanced CVD silicon dioxide or a silicate glass). Each deposited layer of polysilicon or sacrificial material can be patterned using a photolithographically defined mask and etching. Such patterned multi-layer structures have lateral dimensions of 5–500 microns and layer thicknesses of less than a few microns. After the multi-layer structure is built up, the MEMS device can be released by selective wet etching of the sacrificial layers in aqueous hydrofluoric acid (HF). After etching, the released MEMS device structure can be rinsed in deionized water to remove the etchant and etch products.
Due to the large surface area-to-volume ratio of compliant structures, a MEMS device is susceptible to interlayer or layer-to-substrate adhesion during the release process (release adhesion) or subsequent device use (in-use adhesion). This adhesion phenomenon is more generally called stiction. Stiction is exacerbated by the ready formation of a 5–30 angstrom thick native oxide layer on the silicon surface, either during post-release processing of the MEMS device or during subsequent exposure to air during use. Silicon oxide is hydrophilic, encouraging the formation of water layers on the native oxide surfaces that can exhibit strong capillary forces when the small interlayer gaps are exposed to a high humidity environment. Furthermore, Van der Waals forces, due to the presence of certain organic residues; hydrogen bonding; and electrostatic forces also contribute to the interlayer attraction. These cohesive forces can be strong enough to pull the free-standing released layers into contact, causing irreversible latching and yielding the MEMS device inoperative.
Drying techniques, such as freeze-sublimation and supercritical carbon dioxide drying, have been shown to prevent liquid formation during the release process, thereby preventing capillary collapse and release adhesion. Furthermore, stiction can be reduced by minimizing contact surface areas, designing MEMS device structures that are stiff in the out-of-plane direction, and hermetic packaging. However, in-use adhesion remains a serious reliability problem with MEMS devices.
Surface modification is one means to produce low surface energy, hydrophobic surfaces, thereby inhibiting in-use adhesion in released MEMS devices. Any surface modification or coating process must be compatible with subsequent device fabrication processes, including the back-end processes of wafer dicing, die attachment, and hermetic encapsulation. These later packaging processes can require elevated temperatures of up to 500° C. and compatibility with inert gas or vacuum encapsulating environments. Furthermore, a coating should not introduce stress gradients and should coat surfaces that are inaccessible to conventional line-of-sight deposition processes. Roya Maboudian, “Surface processes in MEMS technology,” Surface Science Reports 30, 207 (1998).
Most coating processes have the goal of producing a thin surface layer bound to the native silicon oxide that presents a hydrophobic surface to the environment. In particular, coating the MEMS device surface with self-assembled monolayers (SAMs) having a hydrophobic tail group has been shown to be effective in reducing in-use adhesion. SAMs have typically involved the deposition of organosilane coupling agents, such as octadecyltrichlorosilane and perfluorodecyltrichlorosilane, from nonaqueous solutions after the MEMS device is released.
FIG. 1 shows the formation of a typical chlorosilane-based SAM on a hydroxylated silicon surface. Currently used SAM precursors are deposited via a standard nucleophilic type II (SN2) reaction. Surface self-assembly is thought to occur by a hydrolysis reaction of the chlorosilane coupling agent with water, releasing hydrogen chloride to form hydroxysilanes that hydrogen bond to each other and surface hydroxyl groups to form a dense monolayer on the silicon surface. Upon heating of the hydrogen-bonded monolayer, a condensation reaction is then thought to occur in which the hydroxyl groups react with each other to form siloxane cross-linkages to neighboring silanes within the monolayer and covalent bonds to the oxide surface. The hydrocarbon or fluorocarbon tail groups thereby provide an effective anti-stiction coating on the MEMS device surface.
However, the deposition of chlorosilane-based SAMs from liquid has been shown to be unreliable in practice, due to the need to control trace amounts of water in the non-aqueous deposition solvent, micelle formation in and instability of the precursor solutions, incomplete wetting of high aspect ratio MEMS device structures, disposal of the solvent waste, and degradation of the SAM coatings in high temperature and humid ambients. M. P. de Boer and T. M. Mayer, “Tribology of MEMS,” MRS Bulletin, April 2001, pp. 302–304 and B. C. Bunker et al., Langmuir 16, 7742 (2000).
Vapor phase deposition of anti-stiction coatings has numerous advantages over solution-phase processes, including efficient transport into high-aspect-ratio MEMS device structures, conformal coverage, control of the deposition environment, and avoidance of waste solvents. For the vapor deposition of anti-stiction coatings on the surface of MEMS devices, precursors must be vaporized and then brought into contact with the surface on which the coating forms. However, for vapor-phase deposition of coating materials on MEMS devices, the type of leaving functionality is critical to the effectiveness of the coating process.
For example, vapor-deposited anti-stiction coatings have been synthesized from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS) and water vapor in a low pressure CVD process at room temperature. However, the use of a chloro-functionality has been found to result in incomplete adhesion of the chlorosilane precursor to the silicon dioxide surface. As with solution-phase SAM coatings, chloro-functionalized and hydroxy-functionalized precursors react slowly with the surface and require the addition of water vapor and an anneal step to enable the formation of bonds. Furthermore, the larger precursor molecules are preferred for SAM coatings, since hydrophobicity and thermal stability generally decrease with lower molecular weight. However, to achieve adequate coverage, the chlorosilane precursor should be applied at pressures above 0.1 Torr. This high-pressure requirement makes it impractical to use the same chlorosilane precursors that are presently being used with solution-phase chemical deposition of SAMs because their vapor pressure is too low for effective vapor phase depositions, due to their high molecular weight. T. M. Mayer et al., “Chemical Vapor Deposition of Fluoroalkylsilane Monolayer Films for Adhesion Control in Microelectromechanical Systems,” J. Vac. Sci. Technol. B18, 2433 (2000).
Thus, there remains a need for a reliable anti-stiction coating for MEMS devices that is compatible with MEMS fabrication processes. The present invention provides a method for the CVD of amino-functionalized silane precursors that can be used as anti-stiction coatings on MEMS devices.
The use of the amino-functionality for the silane precursor mitigates many of the problems associated with chlorosilane coupling agents for SAM coatings. For example, aminosilane precursor molecules having an amino leaving-group react immediately with the silicon oxide surface and do not require the addition of water vapor to effectuate the reaction. The process produces direct covalent bonds between the surface and the precursor, and thus eliminates the need for a post-process anneal step. Multiple amino pendants can provide additional reactivity. For example, Si(NMe2)4 reacts with a surface hydroxyl group yielding a covalently bound species with a high density of additional dimethylamino reactive centers. Also, the reaction may be performed at very low pressure (10−5 Torr) which is generally required when using precursors having a high molecular weight.
The vapor-phase deposited anti-stiction coatings of the present invention exhibit more uniform surface morphology and stronger adhesion to the surface than solution-phase SAMs. Vapor-deposited coatings exhibit better lubrication characteristics, adsorb less water, and have longer-term stability. Vapor-deposited coatings also have fabrication advantages as compared to solution-phase SAMs. The vapor deposition process is more easily scalable to full wafers and consumes less solvent and precursor. In addition, the vapor deposition process takes less time, can be more easily automated, and is more repeatable.
The vapor deposition process of the present invention further provides a means for conditioning the silicon surface (or priming the surface) so that a dense precursor coating can be deposited. When the primed surface is treated in an O2/H2 plasma, the resultant surface contains a greater number of hydroxyl groups than the untreated silicon surface. After conditioning, a hydrophobic anti-stiction coating can be deposited at a higher density than occurs with the untreated surface.